Not too sure the speed differences of the G5 1.8 vs the 2.0, check here tho... http://www.apple-history.com/
I noticed a big step in speed going from a G4 1.25 MDD to G5 1.8...and both these macs are single processors. Now have a dual G5 2.0 which is even faster. If you have enuff cash to get a dual, go for it. I am sure you already now how to eek power out of your system, as you kept that G4 for a while, which means youll keep this new one for awhile too. My average is new mac, new to me at least!, every 3 years. YMMV.
My advice, get the fastest Mac you can afford, even if has less ram/HD space, these can be upgraded later.